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Versions: 00 01 02 03 04 05 rfc6319                        Informational
Network Working Group                                         M. Azinger
Internet-Draft                                   Frontier Communications
Intended status: Informational                               Corporation
Expires: July 8, 2011                                          L. Vegoda
                                                         January 4, 2011

Issues Associated with Designating Additional Private IPv4 Address Space


   When a private network or internetwork grows very large it is
   sometimes not possible to address all interfaces using private IPv4
   address space because there are not enough addresses.  This document
   describes the problems faced by those networks, the available options
   and the issues involved in assigning a new block of private IPv4
   address space.

   While this informational document does not make a recommendation for
   action, it documents the issues surrounding the various options that
   have been considered.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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Copyright Notice

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Large Networks . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Non-Unique Addresses . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Subscriber Use Network Address Translation . . . . . . . .  3
     3.2.  Carrier Grade Network Address Translation  . . . . . . . .  4
   4.  Available Options  . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  IPv6 Options . . . . . . . . . . . . . . . . . . . . . . .  4
       4.1.1.  Unique Globally Scoped IPv6 Unicast Addresses  . . . .  4
       4.1.2.  Unique Local IPv6 Unicast Addresses  . . . . . . . . .  4
     4.2.  IPv4 Options . . . . . . . . . . . . . . . . . . . . . . .  5
       4.2.1.  Address Transfers or Leases From Organizations
               with Available Address Space . . . . . . . . . . . . .  5
       4.2.2.  Using Unannounced Address Space Allocated to
               Another Organization . . . . . . . . . . . . . . . . .  5
       4.2.3.  Unique IPv4 Space Registered by an RIR . . . . . . . .  6
   5.  Options and Consequences for Defining New Private Use Space  .  6
     5.1.  Redefining Existing Unicast Space as Private Address
           Space  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.2.  Unique IPv4 Space Shared by a Group of Operators . . . . .  7
     5.3.  Potential Consequences of Not Redefining Existing
           Unicast Space as Private Address Space . . . . . . . . . .  8
     5.4.  Redefining Future Use Space as Unicast Address Space . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   [RFC1918] sets aside three blocks of IPv4 address space for use in
   private networks:, and
   These blocks can be used simultaneously in multiple, separately
   managed networks without registration or coordination with IANA or
   any Internet registry.  Very large networks can find that they need
   to number more device interfaces than there are available addresses
   in these three ranges.  It has occasionally been suggested that
   additional private IPv4 address space should be reserved for use by
   these networks.  Although such an action might address some of the
   needs for these very large network operators it is not without
   consequences, particularly as we near the date when the IANA free
   pool will be fully allocated.

2.  Large Networks

   The main categories of very large networks using private address
   space are: cable operators, wireless (cell phone) operators, private
   internets and VPN service providers.  In the case of the first two
   categories, the complete address space reserved in [RFC1918] tends to
   be used by a single organization.  In the case of private internets
   and VPN service providers there are multiple independently managed
   and operated networks and the difficulty is in avoiding address

3.  Non-Unique Addresses

3.1.  Subscriber Use Network Address Translation

   The address space set aside in [RFC1918] is a finite resource which
   can be used to provide limited Internet access via Network Address
   Translation (NAT).  A discussion of the advantages and disadvantages
   of NATs is outside the scope of this document but a an analysis of
   the advantages, disadvantages and architectural implications can be
   found in [RFC2993].  Nonetheless, it must be acknowledged that NAT is
   adequate in some situations and not in others.  For instance, it
   might technically feasible to use NAT or even multiple layers of NAT
   within the networks operated by residential users or corporations
   where only limited Internet access is required.  A more detailed
   analysis can be found in [RFC3022].  Where true peer to peer
   communication is needed or where services or applications do not work
   properly behind NAT, globally unique address space is required.  In
   other cases, NAT traversal techniques facilitate peer-to-peer like
   communication for devices behind NATs.

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   In many cases it is possible to use multiple layers of NAT to re-use
   parts of the address space defined in [RFC1918].  It is not always
   possible to rely on CPE devices using any particular range, however.
   In some cases this means that unorthodox workarounds including
   assigning CPE devices unallocated address space or address space
   allocated to other network operators are feasible.  In other cases,
   organizations choose to operate multiple separate routing domains to
   allow them to re-use the same private address ranges in multiple
   contexts.  One consequence of this is the added complexity involved
   in identifying which system is referred to when an IP address is
   identified in a log or management systems.

3.2.  Carrier Grade Network Address Translation

   Another option is to share one address across multiple interfaces and
   in some cases, subscribers.  This model breaks the classical model
   used for logging address assignments and creates significant risks
   and additional burdens, as described in [CLAYTON] and more fully
   discussed in [FORD] and is documented in [DS-LITE].

4.  Available Options

   When a network operator has exhausted the private address space set
   aside in [RFC1918] but needs to continue operating a single routing
   domain a number of options are available.  These include:

4.1.  IPv6 Options

4.1.1.  Unique Globally Scoped IPv6 Unicast Addresses

   Using unique, globally scoped IPv6 unicast addresses is the best
   permanent solution as it removes any concerns about address scarcity
   within the next few decades.  Implementing IPv6 is a major endeavor
   for service providers with millions of consumer customers and is
   likely to take considerable effort and time.  In some cases
   implementing a new network protocol on a very large network takes
   more time than is available, based on network growth and the
   proportion of private space that has already been used.  In these
   cases, there is a call for additional private address space that can
   be shared by all network operators.  [DAVIES] makes one such case.

4.1.2.  Unique Local IPv6 Unicast Addresses

   Using the unique, local IPv6 unicast addresses defined in [RFC4193]
   is another approach and does not require coordination with an
   Internet registry.  Although the addresses defined in [RFC4193] are
   probabilistically unique, network operators on private internets and

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   those providing VPN services might not want to use them because there
   is a very low probability of non-unique locally assigned global IDs
   being generated by the algorithm.  Also, in the case of private
   internets, it can be very challenging to coordinate the introduction
   of a new network protocol to support the internet's continued growth.

4.2.  IPv4 Options

4.2.1.  Address Transfers or Leases From Organizations with Available
        Address Space

   The Regional Internet Registry (RIR) communities have recently been
   developing policies to allow organizations with available address
   space to transfer such designated space to other organizations
   [RIR-POLICY].  In other cases, leases might be arranged.  This
   approach is only viable for operators of very large networks if
   enough address space is made available for transfer or lease and if
   the very large networks are able to pay the costs of these transfers.
   It is not possible to know how much address space will become
   available in this way, when it will be available and how much it will
   cost.  However, it is unlikely to become available in large
   contiguous blocks and this would add to the network management burden
   for the operator as a significant number of small prefixes would
   inflate the size of the operators routing table at a time when it is
   also adding an IPv6 routing table.  These reasons will make address
   transfers a less attractive proposition to many large network
   operators.  Leases might not be attractive to some organizations if
   both parties cannot agree a suitable length of time.  Also, the
   lessor might worry about its own unanticipated needs for additional
   IPv4 address space.

4.2.2.  Using Unannounced Address Space Allocated to Another

   Some network operators have considered using IP address space which
   is allocated to another organization but is not publicly visible in
   BGP routing tables.  This option is very strongly discouraged as the
   fact that an address block is not visible from one view does not mean
   that it is not visible from another.  Furthermore, address usage
   tends to leak beyond private network borders in e-mail headers, DNS
   queries, traceroute output and other ways.  The ambiguity this causes
   is problematic for multiple organizations.  This issue is discussed
   in [RFC3879], section 2.3.

   It is also possible that the registrant of the address block might
   want to increase its visibility to other networks in the future,
   causing problems for anyone using it unofficially.  In some cases
   there might also be legal risks involved in using address space

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   officially allocated to another organization.

   Where this has happened in the past it has caused operational
   problems [FASTWEB].

4.2.3.  Unique IPv4 Space Registered by an RIR

   RIRs policies allow network operators to receive unique IP addresses
   for use on internal networks.  Further, network operators are not
   required to have already exhausted the private address space set
   aside in [RFC1918].  Nonetheless, network operators are naturally
   disinclined to request unique IPv4 addresses for the private areas of
   their networks as using addresses in this way means they are not
   available for use by new Internet user connections.

   It is likely to become more difficult for network operators to obtain
   large blocks of unique address space as we approach the point where
   all IPv4 unicast /8s have been allocated.  Several RIRs already have
   policies how to allocate from their last /8 [RIR-POLICY-FINAL-8] and
   there have been policy discussions that would reduce the maximum
   allocation size available to network operators [MAX-ALLOC] or would
   reduce the period of need for which the RIR can allocate

5.  Options and Consequences for Defining New Private Use Space

5.1.  Redefining Existing Unicast Space as Private Address Space

   It is possible to re-designate a portion of the current global
   unicast IPv4 address space as private unicast address space.  Doing
   this could benefit a number of operators of large network for the
   short period before they complete their IPv6 roll-out.  However, this
   benefit incurs a cost by reducing the pool of global unicast
   addresses available to users in general.

   When discussing re-designating a portion of the current global
   unicast IPv4 address space as private unicast address space it is
   important to consider how much space would be used and for how long
   it would be sufficient.  Not all of the large networks making full
   use of the space defined in [RFC1918] would have their needs met with
   a single /8.  In 2005, [HAIN] suggested reserving three /8s for this
   purpose while in 2009 [DAVIES] suggested a single /10 would be
   sufficient.  There does not seem to be a consensus for a particular
   prefix length nor an agreed basis for deciding what is sufficient.
   The problem is exacerbated by the continually changing needs of ever
   expanding networks.

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   A further consideration is which of the currently unallocated IPv4
   unicast /8 blocks should be used for this purpose.  Using address
   space which is known to be used unofficially is tempting.  For
   instance,, which was unallocated until January 2010, was
   proposed in [HAIN] and is known to be used by a number of different
   users.  These include networks making use of HIP LSIs [RFC4423],
   [WIANA], [anoNet] and others.  There is anecdotal [VEGODA] and
   research [WESSELS] evidence to suggest that several other IPv4 /8s
   are used in this fashion.  Also there have been discussions [NANOG]
   about some sections of these /8's being carved out and filtered
   therefore unofficially enabling the use of these sections for private

   Although new IPv4 /8s are allocated approximately once a month, they
   are not easy to bring into use because network operators are slow to
   change their filter configurations.  This is despite long-running
   awareness campaigns [CYMRU], [LEWIS] and active work [ripe-351] to
   notify people whose filters are not changed in a timely fashion.
   Updating code that recognises private address space in deployed
   software and infrastructure systems is likely to be far more
   difficult as many systems have these ranges hard-coded and cannot be
   quickly changed with a new configuration file.

   Another consideration when redefining existing unicast space as
   private address space is that no single class of user can expect the
   space to stay unique to them.  This means that an ISP using a new
   private address range cannot expect its customers not to already be
   using that address range within their own networks.

5.2.  Unique IPv4 Space Shared by a Group of Operators

   Where a group of networks find themselves in a position where they
   each need a large amount of IPv4 address space from an RIR in
   addition to that defined in [RFC1918] they might cooperatively agree
   to all use the same address space to number their networks.  The
   clear benefit to this approach is that it significantly reduces the
   potential demand on the pool of unallocated IPv4 address space.
   However, the issues discussed in 4.4 could also be of concern here,
   particularly the possibility that one operator might decide to use
   the address space to number customer connections, rather than private

   Nonetheless, this approach has the potential to create an unofficial
   new private address range without proper scrutiny.

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5.3.  Potential Consequences of Not Redefining Existing Unicast Space as
      Private Address Space

   If additional private address space is not defined and the large
   network operators affected by this problem are not able to solve
   their problems with IPv6 address space or by segmenting their
   networks into multiple routing domains, those networks will need
   unique IPv4 addresses.  It is possible and even likely that a single
   network could consume a whole IPv4 /8 in a year.  At the time of
   writing there are just 24 unallocated IPv4 /8s, so it would not take
   many such requests to make a major dent in the available IPv4 address
   space.  [POTAROO] provides an analysis of IPv4 address consumption
   and projects the date on which the IANA and RIR pools will be fully

5.4.  Redefining Future Use Space as Unicast Address Space

   There have also been proposals to re-designate the former Class E
   space ( as unicast address space.  [WILSON] suggests that
   it should be privately scoped while [FULLER] does not propose a
   scope.  Both proposals note that existing deployed equipment may not
   be able to use addresses from  Potential users would
   need to be sure of the status of the equipment on their network and
   the networks with which they intend to communicate.

   It is not immediately clear how useful could be in
   practice.  While [FULLER] documents the status of several popular
   desktop and server operating systems, the status of the most widely
   deployed routers and switches is less clear and it is possible that might only be useful in very large, new green field
   deployments where full control of all deployed systems is available.
   However, in such cases it might well be easier to deploy an IPv6

6.  Security Considerations

   This document has no security implications.

7.  IANA Considerations

   This document makes no request of IANA.

8.  References

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8.1.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
              November 2000.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

8.2.  Informative References

   [RFC3879]  Huitema, C. and B. Carpenter, "Deprecating Site Local
              Addresses", RFC 3879, September 2004.

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, May 2006.

   [anoNet]   anoNet, "anoNet: Cooperative Chaos",

   [CLAYTON]  Clayton, R., "Practical mobile Internet access
              traceability", January 2010, <http://

   [CYMRU]    Greene, B., "The Bogon Reference",

   [DAVIES]   Davies, G. and C. Liljenstolpe, "Work in Progress:
              Transitional non-conflicting reusable IPv4 address block",
              November 2009, <http://tools.ietf.org/html/

   [DS-LITE]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Work in
              Progress: Dual-Stack Lite Broadband Deployments Following
              IPv4 Exhaustion", August 2010, <http://tools.ietf.org/

   [FASTWEB]  Aina, A., "41/8 announcement", May 2006,

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   [FORD]     Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
              Roberts, "Work in Progress: Issues with IP Address
              Sharing", March 2010, <http://tools.ietf.org/html/

   [FULLER]   Fuller, V., Lear, E., and D. Meyer, "Work in Progress:
              Reclassifying 240/4 as usable unicast address space",
              March 2008,

   [HAIN]     Hain, T., "Work in Progress: Expanded Address Allocation
              for Private Internets", January 2005,

   [LEWIS]    Lewis, J., "This system has been setup for testing
              purposes for 69/8 address space", March 2003,

              Spenceley, J. and J. Martin, "prop-070: Maximum IPv4
              allocation size", January 2009,

   [NANOG]    Dickson, B., "1/8 and 27/8 allocated to APNIC",
              January 2010, <http://mailman.nanog.org/pipermail/nanog/

   [POTAROO]  Huston, G., "IPv4 Address Report",

              Karrenberg, D., "De-Bogonising New Address Blocks",
              October 2005,

              Number Resource Organization, "RIR Comparative Policy
              Overview, October 2009, Section 1.3.2 Transfer of

              Number Resource Organization, "RIR Comparative Policy
              Overview, October 2009, 2.6. Use of Final Unallocated IPv4
              Address Space", October 2009,


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              Karrenberg, D., O'Reilly, N., Titley, N., and R. Bush,
              "RIPE Policy Proposal 2009-03", April 2009,

   [VEGODA]   Vegoda, L., "Awkward /8 Assignments", September 2007, <htt

   [WESSELS]  Wessels, D., "Searching for Evidence of Unallocated
              Address Space Usage in DITL 2008 Data", June 2008, <https:

   [WIANA]    WIANA, "The Wireless Internet Assigned Numbers Authority",

   [WILSON]   Wilson, P., Michaelson, G., and G. Huston, "Work in
              Progress: Redesignation of 240/4 from "Future Use" to
              "Private Use"",

Appendix A.  Acknowledgments

   The authors would also like to thank Ron Bonica, Michelle Cotton, Lee
   Howard and Barbara Roseman for their assistance in early discussions
   of this document and to Maria Blackmore, Alex Bligh, Mat Ford, Thomas
   Narten, Ricardo Patara and for improvement suggestions.

Authors' Addresses

   Marla Azinger
   Frontier Communications Corporation
   Vancouver, WA
   United States of America

   Email: marla.azinger@ftr.com
   URI:   http://www.frontiercorp.com/

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   Leo Vegoda
   Internet Corporation for Assigned Names and Numbers
   4676 Admiralty Way, Suite 330
   Marina del Rey, CA  90292
   United States of America

   Phone: +1-310-823-9358
   Email: leo.vegoda@icann.org
   URI:   http://www.iana.org/

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